This invention relates generally to television signal scrambling systems and in particular to method and apparatus for enhancing the sound masking in a phase reversal television scrambling system.
As fully described in the above-mentioned related patent, a technique for scrambling television signals involves synchronizing signal (sync) suppression and carrier phase reversal during the horizontal blanking intervals. This technique, with the appropriate use of a synchronous detector, enables recovery of the scrambled television signal by detection of the carrier phase reversal points. In using such a scrambling technique with television receivers having intercarrier sound, a series of harmonics of the carrier reversal frequency is developed during detection by an unauthorized receiver because of the phase reversals. If the carrier reversals are at the horizontal rate, which is the preferred mode of practicing the invention of the above-mentioned related patent, these spurious signals occur at 1 H, 2 H, 3 H, 4 H, 5 H, etc. where H is equal to the horizontal line frequency (approximately 15.75 KHz). It will be appreciated, that if a proper decoder is utilized for receiving and decoding the signal, the carrier phase reversals are corrected and the spurious signals are not present.
The present invention takes advantage of the fact that spurious signals at harmonics of the carrier phase reversal frequency will be present unless the scrambled signal is properly unscrambled, and adds an additional "sound signal" to create interference for masking the television program sound when the signal is received by an unauthorized receiver. The sound masking that occurs is useful for two reasons, one being that in many instances a significant benefit is obtained by unauthorized subscribers merely receiving the audio portion of a scrambled television signal. The other, and more important, reason, is that in many instances material that might be considered offensive by many listeners is garbled to the point of unintelligibility. The audio masking in the scrambling system of the invention has the added advantage that it entails no additional cost in the decoder.